Bend-resistant multimode optical fiber

ABSTRACT

A multimode optical fiber includes a central core surrounded by an outer cladding. The central core has a graded-index profile with respect to the outer cladding and an outer radius r 1  of between about 30 microns and 50 microns (e.g., between about 35 microns and 45 microns). The optical fiber also includes an inner cladding positioned between the central core and the outer cladding, and a depressed trench positioned between the inner cladding and the outer cladding. The multimode optical fiber exhibits reduced bending losses.

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This application is a continuation of commonly assigned U.S. patent application Ser. No. 13/428,520 for a “Bend-Resistant Multimode Optical Fiber” (filed Mar. 23, 2012, and published Sep. 27, 2012, as Publication No. 2012/0243843 A1), now U.S. Pat. No. 9,341,771.

Parent U.S. patent application Ser. No. 13/428,520 claims the benefit of European Application No. 11305328.4 for a “Multimode Optical Fiber with Improved Bend Resistance” (filed Mar. 24, 2011, at the European Patent Office).

Parent U.S. patent application Ser. No. 13/428,520 further claims the benefit of commonly assigned U.S. Provisional Patent Application Ser. No. 61/511,672 for a “Multimode Optical Fiber with Improved Bend Resistance” (filed Jul. 26, 2011).

Each of the foregoing patent applications and patent application publication is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of fiber optic transmission and, more specifically, to a multimode optical fiber having reduced bending losses.

BACKGROUND

An optical fiber (i.e., a glass fiber typically surrounded by one or more coating layers) conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core n_(c) is typically greater than the refractive index of the optical cladding n_(g) (i.e., n_(c)>n_(g)).

Multimode optical fibers are commonly used for short-distance applications requiring a broad bandwidth, such as local networks or LAN (local area network). The core of a multimode optical fiber typically has a diameter of between about 50 microns and 62.5 microns, whereas the core of a single-mode optical fiber typically has a diameter of between about 6 microns and 9 microns. In a multimode optical fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber.

Multimode optical fibers have been the subject of international standardization under the ITU-T G.651.1 recommendations, which, in particular, define criteria (e.g., bandwidth, numerical aperture, and core diameter) that relate to the requirements for optical fiber compatibility. The ITU-T G.651.1 standard (July 2007) is hereby incorporated by reference in its entirety.

The numerical aperture (NA) of an optical fiber can be approximated by the following equation: NA=√{square root over (n _(c) ² −n _(g) ²)} where n_(c) is the refractive index of the central core and n_(g) is the refractive index of the outer cladding (e.g., an outer optical cladding).

For optical fibers, the refractive index profile is generally classified according to the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding (e.g., an outer optical cladding) is shown on the y-axis. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, “parabolic” profile, or “triangular” profile for graphs having the respective shapes of a step, a trapezoid, a parabola, or a triangle. These curves are generally representative of the optical fiber's theoretical or set profile. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.

For the same propagation medium (i.e., in a step-index multimode optical fiber), the different modes have different group delay times. This difference in group delay times results in a time lag (i.e., a delay) between the pulses propagating along different radial offsets of the optical fiber. This delay causes a broadening of the resulting light pulse. Broadening of the light pulse increases the risk of the pulse being superimposed onto a trailing pulse, which reduces the bandwidth (i.e., data rate) supported by the optical fiber. The bandwidth, therefore, is linked to the group delay time of the optical modes propagating in the multimode core of the optical fiber. Thus, to guarantee a broad bandwidth, it is desirable for the group delay times of all the modes to be identical. Stated differently, the intermodal dispersion should be zero, or at least minimized, for a given wavelength.

To reduce intermodal dispersion, the multimode optical fibers used in telecommunications generally have a core with a refractive index that decreases progressively from the center of the optical fiber to its interface with a cladding (i.e., an “alpha” core profile). Such an optical fiber has been used for a number of years, and its characteristics have been described in “Multimode Theory of Graded-Core Fibers” by D. Gloge et al., Bell system Technical Journal 1973, pp. 1563-1578, and summarized in “Comprehensive Theory of Dispersion in Graded-Index Optical Fibers” by G. Yabre, Journal of Lightwave Technology, February 2000, Vol. 18, No. 2, pp. 166-177. Each of the above-referenced articles is hereby incorporated by reference in its entirety.

A graded-index profile (i.e., an alpha-index profile) can be described by a relationship between the refractive index value n and the distance r from the center of the optical fiber according to the following equation:

$\begin{matrix} {n = {n_{1}\sqrt{1 - {2\;{\Delta\left( \frac{r}{a} \right)}^{\alpha}}}}} & \left( {{Equation}\mspace{14mu} 1} \right) \end{matrix}$ wherein,

α≦1, and α is a non-dimensional parameter that is indicative of the shape of the index profile;

n₁ is the maximum refractive index of the optical fiber's core;

a is the radius of the optical fiber's core; and

$\begin{matrix} {\Delta = \frac{\left( {n_{1}^{2} - n_{0}^{2}} \right)}{2\; n_{1}^{2}}} & \left( {{Equation}\mspace{14mu} 2} \right) \end{matrix}$

where n₀ is the minimum refractive index of the multimode core, which may correspond to the refractive index of the outer cladding (most often made of silica).

A multimode optical fiber with a graded index (i.e., an alpha profile) therefore has a core profile with a rotational symmetry such that along any radial direction of the optical fiber the value of the refractive index decreases continuously from the center of the optical fiber's core to its periphery. When a multimode light signal propagates in such a graded-index core, the different optical modes experience differing propagation mediums (i.e., because of the varying refractive indices). This, in turn, affects the propagation speed of each optical mode differently. Thus, by adjusting the value of the parameter α, it is possible to obtain a group delay time that is virtually equal for all of the modes. Stated differently, the refractive index profile can be modified to reduce or even eliminate intermodal dispersion.

Typically, multimode optical fibers with higher numerical apertures have lower macrobending losses (referred to hereafter as “bending losses”).

Conventional multimode optical fibers having a central core diameter of more than 50 microns are generally expected to provide sufficient bend resistance for many applications. Such exemplary optical fibers may have a central core diameter of 62.5 microns and a numerical aperture of 0.275 or a central core diameter of 80 microns and a numerical aperture of 0.3.

Nevertheless, for tighter bend radii (e.g., 5 millimeters), such optical fibers exhibit significant bending losses that may be critical for high speed transmission (e.g., in compact consumer electronic devices).

International Publication No. WO 2010/036684, which is hereby incorporated by reference in its entirety, deals with large-core optical fibers. The disclosed optical fibers, however, have a central-core radius a and a relative refractive index difference Δ such that:

$\frac{\sqrt{2\Delta}}{a} < {5.1 \times 10^{- 3}\mspace{14mu}{{µm}^{- 1}.}}$

The central cores of the disclosed optical fibers fail to provide reduced microbending losses, because, for a given Δ value, enlarging the central core will result in larger microbending losses. Furthermore, the relationships between the disclosed central-core radius a and the relative refractive index difference Δ lead to undesirably large microbending losses.

Therefore, a need exists for a multimode optical fiber having reduced bending losses and a central-core diameter of greater than 50 microns.

SUMMARY

Accordingly, in one aspect, the present invention embraces an optical fiber that includes a central core surrounded by an outer cladding (e.g., an outer optical cladding). The outer cladding has a refractive index value n_(c1). The central core has an outer radius r₁, a maximum refractive index value n₀, and a graded-index profile with respect to the outer cladding. The central core also has a relative refractive index difference:

$\Delta = {\frac{n_{0}^{2} - n_{cl}^{2}}{2\; n_{0}^{2}}.}$ The central core's outer radius r₁ is typically between about 30 microns and 50 microns (e.g., 35 microns to 50 microns). Typically, the central core's outer radius r₁ and relative refractive index difference Δ satisfy the following inequality:

$\frac{\sqrt{2\Delta}}{r_{1}} \geq {5.1 \times 10^{- 3}\mspace{14mu}{{µm}^{- 1}.}}$

An inner cladding is positioned between the central core and the outer cladding (e.g., immediately surrounding the central core). The inner cladding has an outer radius r₂ and a refractive index difference Δn₂ with respect to the outer cladding. Typically, the difference r₂−r₁ between the inner cladding's outer radius r₂ and the central core's outer radius r₁ is between about 1 micron and 3 microns.

A depressed trench is positioned between the inner cladding and the outer cladding (e.g., immediately surrounding the inner cladding). The depressed trench has an outer radius r₃ and a negative refractive index difference Δn₃ with respect to the outer cladding. Typically, the difference r₃−r₂ between the depressed trench's outer radius r₃ and the inner cladding's outer radius r₂ is between about 3 microns and 7 microns. The depressed trench's refractive index difference Δn₃ is typically between about −10×10⁻³ and −5×10⁻³.

In an exemplary embodiment, for two turns around a radius of curvature of 5 millimeters at a wavelength of 850 nanometers, the optical fiber has bending losses of less than about 0.3 dB.

In another exemplary embodiment, for two turns around a radius of curvature of 5 millimeters at a wavelength of 850 nanometers, the optical fiber has bending losses of less than about 0.2 dB.

In yet another exemplary embodiment, the central core's outer radius r₁ and relative refractive index difference Δ satisfy the following inequality:

$\frac{\sqrt{2\Delta}}{r_{1}} \geq {5.4 \times 10^{- 3}\mspace{14mu}{{µm}^{- 1}.}}$

In yet another exemplary embodiment, the central core's outer radius r₁ is between about 35 microns and 50 microns (e.g., between about 35 microns and 45 microns).

In yet another exemplary embodiment, the central core's outer radius r₁ is greater than about 36 microns (e.g., about 37 microns or greater).

In yet another exemplary embodiment, the central core's outer radius r₁ is greater than about 38 microns (e.g., between about 39 microns and 41 microns).

In yet another exemplary embodiment, the central core's outer diameter (i.e., 2r₁) is about 62.5 microns.

In yet another exemplary embodiment, the central core's outer diameter (i.e., 2r₁) is about 80 microns (i.e., outer radius r₁ is about 40 microns).

In another aspect, the present invention embraces a multimode optical system that includes an optical fiber in accordance with the foregoing.

The foregoing illustrative summary, as well as other exemplary objectives and/or advantages of the invention, and the manner in which the same are accomplished, are further explained within the following detailed description and its accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically depicts the refractive index profile of an exemplary optical fiber according to the present invention.

FIG. 2 graphically depicts bending losses as a function of bending radius (i.e., radius of curvature) for an exemplary optical fiber according to the present invention and a comparative optical fiber.

FIG. 3 graphically depicts bending losses as a function of bending radius (i.e., radius of curvature) for another exemplary optical fiber according to the present invention and another comparative optical fiber.

FIG. 4 depicts an optical-fiber cable undergoing a pinch test.

DETAILED DESCRIPTION

The present invention embraces a multimode optical fiber having reduced bending losses and a central-core diameter of greater than 50 microns.

FIG. 1 depicts the refractive index profile of an exemplary optical fiber in accordance with the present invention. The exemplary optical fiber is a multimode optical fiber that includes a central core (e.g., an inner core) surrounded by an outer cladding (e.g., an outer optical cladding).

As depicted, the exemplary optical fiber also includes an inner cladding positioned between the central core and the outer cladding (e.g., immediately surrounding the central core). A depressed trench is positioned between the inner cladding and the outer optical cladding (e.g., immediately surrounding the inner cladding).

The central core has an outer radius r₁ that is typically between 30 microns and 50 microns, more typically 35 microns or greater (e.g., between about 39 microns and 41 microns). The central core also has a graded-index profile relative to the outer cladding. The central core's outer radius r₁ and relative refractive index difference Δ typically satisfy the following inequality:

$\frac{\sqrt{2\Delta}}{r_{1}} \geq {5.1 \times 10^{- 3}\mspace{14mu}{{µm}^{- 1}.}}$

The central core's relative refractive index difference Δ is defined by the following equation:

${\Delta = \frac{n_{0}^{2} - n_{cl}^{2}}{2\; n_{0}^{2}}},$ where n₀ is the maximum refractive index value of the central core (typically corresponding to the refractive index value of the center of the central core) and n_(c1) is the refractive index value of the outer cladding.

The optical fiber's inner cladding has an outer radius r₂. Typically, the difference r₂−r₁ between the inner cladding's outer radius r₂ and the central core's outer radius r₁ (e.g., the radial distance between the central core's outer radius and the inner radius of the depressed trench) is between 1 micron and 3 microns (e.g., about 2 microns or less).

The depressed trench has an outer radius r₃ and a negative refractive index difference Δn₃ with respect to the outer cladding. Typically, the difference r₃−r₂ between the depressed trench's outer radius r₃ and the inner cladding's outer radius r₂ (e.g., the width of the depressed trench) is between 3 microns and 7 microns (e.g., about 5 microns or less, such as about 4 microns). The depressed trench's refractive index difference Δn₃ (e.g., the depth of the depressed trench) is typically between −10×10⁻³ and −5×10⁻³.

The present multimode optical fiber provides excellent macrobending performance. For example, for two turns around a radius of curvature of 5 millimeters at a wavelength of 850 nanometers, the optical fiber typically has bending losses of less than 0.3 dB (e.g., less than 0.2 dB). In another exemplary embodiment, for one turn around a radius of curvature of 3 millimeters at a wavelength of 850 nanometers, the optical fiber has bending losses of less than about 0.5 dB.

Exemplary multimode optical fibers in accordance with the present invention include a central core having an outer diameter of 62.5 microns (i.e., 62.5-micron MMFs) and exhibit improved bend resistance. Additionally, exemplary multimode optical fibers include a central core having an outer diameter of 80 microns (i.e., 80-micron MMFs) and exhibit improved bend resistance.

As noted, exemplary multimode optical fibers include a central core having a graded-index profile relative to the outer cladding (i.e., Graded-Index MultiMode Fibers or GI-MMFs). The modal structure of GI-MMFs is as follows.

In multimode optical fibers with the following refractive index profile:

$\begin{matrix} {{n(r)} = \left\{ {\begin{matrix} {n_{1} \cdot \sqrt{1 - {2{\Delta \cdot \left( \frac{r}{a} \right)^{\alpha}}}}} & {{{for}\mspace{14mu} r} \leq a} \\ {n_{1} \cdot \sqrt{1 - {2\Delta}}} & {{{for}\mspace{14mu} r} \geq a} \end{matrix},} \right.} & \left( {{Equation}\mspace{14mu} 3} \right) \end{matrix}$ where α is between about 1.9 and 2.1, Δ>1.2 percent and a>30 microns, the effective indices of the guided modes can be fairly approximated as follows:

$\begin{matrix} {\beta = {{kn}_{1} \cdot \left\lbrack {1 - {2{\Delta\left( \frac{m}{M} \right)}^{{\alpha/\alpha} + 2}}} \right\rbrack^{\frac{1}{2}}}} & \left( {{Equation}\mspace{14mu} 4} \right) \end{matrix}$ where m is the order of the mode, and

${M = {{akn}_{1} \cdot \sqrt{\Delta \cdot \frac{\alpha}{\alpha + 2}}}},$ where

${k = \frac{2\pi}{\lambda}},$ and Δ=850 nanometers.

For a approximately equal to 2, the spacing between modes is somewhat independent from the mode order m and is only a function of the central core's radius a and the central core's height Δ:

$\begin{matrix} {{{\Delta\beta}} \propto \frac{\sqrt{2\Delta}}{a}} & \left( {{Equation}\mspace{14mu} 5} \right) \end{matrix}$

For a 50-micron GI-MMF with Δ=0.9 percent and a=26 microns (as a threshold) this mode spacing (i.e., Δβ) is equal to 5.1×10⁻³ μm⁻¹. Acceptable tolerances with respect to core diameter (e.g., ±2.5 microns) and numerical aperture (e.g., ±0.015) may be taken into account in view of constraints in the manufacturing process.

Without being bound to any particular theory, the present inventors have found that mode spacing affects microbending losses. Indeed, considering the three examples of Table 1, larger values of √{square root over (2Δ/a)} (i.e., the mode-spacing criterion) correlate with lower microbending losses. In Table 1, the relative refractive index difference Δ is given as a percentage.

TABLE 1 Primary coating r₁ Criterion Microbending losses thickness Δ (%) (μm) (μm⁻¹) @ 850 nm (dB/km) (μm) 1.928 38.8 5.06 × 10⁻³ 27.5 27.935 2.059 39.2 5.18 × 10⁻³ 20.3 27.93 2.601 38.4 5.94 × 10⁻³ 12.1 27.75

As shown in Table 1, an increase in the criterion from 5.06×10⁻³ to 5.94×10⁻³ reduces microbending losses two-fold. Accordingly, exemplary optical fibers in accordance with the present invention have a central-core outer radius r₁ and relative refractive index difference Δ such that √{square root over (2Δ/r₁)} is greater than or equal to 5.1×10⁻³ μm⁻¹ (e.g., greater than or equal to 5.2×10⁻³ μm⁻¹), such as greater than or equal to 5.4×10⁻³ μm⁻¹ (e.g., greater than or equal to 5.6×10⁻³ μm⁻¹). Additionally, exemplary optical fibers may have a central-core outer radius r₁ and relative refractive index difference Δ such that √{square root over (2Δ/r₁)} is greater than or equal to 5.7×10⁻³ μm⁻¹ (e.g., greater than or equal to 5.9×10⁻³ μm⁻¹)

Microbending may be analyzed according to the IEC fixed-diameter sandpaper drum test (i.e., IEC TR62221, Method B, 40-micron grade sandpaper), which provides a microbending stress situation that affects multimode fibers even at room temperature. The IEC TR62221 microbending-sensitivity technical report and standard test procedures, including IEC TR62221, Method B (fixed-diameter sandpaper drum) and Method D (basketweave), are hereby incorporated by reference in their entirety.

To satisfy the foregoing relationship between the central core's outer radius r₁ and relative refractive index difference Δ, larger central-core radii demand larger values for Δ, as illustrated in Table 2. In Table 2, the relative refractive index difference Δ is given as a percentage.

TABLE 2 a (μm) Δ (%) NA 25 >0.81 >0.185 31.25 >1.27 >0.232 40 >2.08 >0.297 50 >3.25 >0.373

Typically, macro-bending losses are lower for optical fibers with such large numerical apertures. Furthermore, as illustrated in FIGS. 2 and 3, including a depressed trench between the central core and the outer cladding further improves the macro-bending behavior of these large core radii MMFs.

FIG. 2 graphically depicts bending losses as a function of bending radius (i.e., radius of curvature) for an exemplary trench-assisted optical fiber according to the present invention (i.e., Example 2) and a comparative optical fiber in accordance with the ITU-T G.651.1 recommendations (i.e., Example 1).

FIG. 3 graphically depicts bending losses as a function of bending radius (i.e., radius of curvature) for another exemplary trench-assisted optical fiber according to the present invention (i.e., Example 4) and another comparative optical fiber under a given Encircled Flux Launch (EFL) (i.e., Example 3).

The Encircled Flux (EF) in the optical fibers before bending fulfills the following criteria of Table 3:

TABLE 3 radius (μm) EF min EF max 10 0.2185 0.0635 15 0.38 0.2215 20 0.5249 0.4999

For 62.5-micron and 50-micron MFFs, the conditions on the EF for macrobending loss measurements are given by the ITU-T G.651.1 recommendations and the EF templates within the IEC 61280-4-1 documents, which are hereby incorporated by reference in their entireties.

The comparative optical (Examples 1 and 3) and the exemplary optical fibers (Examples 2 and 4) of FIGS. 2 and 3 correspond to optical fibers having the parameters listed in Table 4.

TABLE 4 Bending losses Trench width Trench depth 2 turns 5 mm Examples a (μm) NA (μm) (×10⁻³) (dB) 1 31.25 0.27 none none 0.739 2 31.25 0.27 4 −8 0.138 3 41 0.31 none none 1.060 4 41 0.31 4 −8 0.253

As noted, Examples 2 and 4 are optical fibers according to the present invention that have a depressed trench of an appropriate width (i.e., r₃−r₂) and depth (i.e., Δn₃). Examples 1 and 3 are comparative optical fibers lacking a depressed trench. The bending losses are measured at a wavelength of 850 nanometers.

As demonstrated by Table 4 and depicted in FIGS. 2 and 3, the depressed trench facilitates the achievement of reduced bending losses. Indeed, the exemplary optical fibers exhibited bending losses that were more than four times less than the bending losses of the comparative optical fibers (i.e., a reduction to less than one-quarter of the comparative optical fibers' bending losses).

Advantages of large-core optical fibers having reduced bending losses are further described in “80 μm-core graded-index MMF for consumer electronic devices” by D. Molin, M. Bigot-Astruc, and P. Sillard, Optoelectronic Interconnects XII, Feb. 2, 2012, Proceedings of SPIE Vol. 8267, which is hereby incorporated by reference in its entirety.

Exemplary optical fibers according to the present invention include depressed trenches having particular volumes V₃. As used herein, the volume V of a depressed trench is defined by the following equation:

V = 2π × ∫_(r_(int))^(r_(ext))Δ  %(r) × r× 𝕕r in which r_(int) and r_(ext) are the inner radius and outer radius of the depressed trench, respectively, and Δ % (r) is the depressed trench's refractive index difference with respect to the outer cladding expressed in terms of percentage. Those of ordinary skill in the art will recognize that this equation can be used for both rectangular and non-rectangular trenches.

If a depressed trench has a rectangular shape (i.e., a step index profile), the equation (above) can be simplified to the following equation: V=Δ%×π×(r _(ext) ² −r _(int) ²) in which r_(ext) and r_(int) are the outer radius and inner radius of the depressed trench, respectively, and Δ% is the depressed trench's refractive index difference with respect to the outer cladding expressed as a percentage.

In this regard, exemplary optical fibers include a depressed trench having a volume V₃ of between about −1710%-μm² and −210%-μm² (e.g., a volume V₃ of about −830%-μm²). Typically, the depressed trench's volume V₃ is about −790%-μm² or greater (e.g., a volume V₃ of about −750%-μm² or greater). More typically, the depressed trench's volume V₃ is about −710%-μm² or greater (e.g., a volume V₃ of about −670%-μm² or greater, such as about −635%-μm² or greater). In exemplary embodiments, the depressed trench's volume V₃ is between about −750%-μm² and −450%-μm² (e.g., between about −628%-μm² and −565%-μm², such as about −600%-μm²).

An optical-fiber cable that includes one more optical fibers in accordance with the present invention may be subjected to a pinch test as depicted in FIG. 4. During the pinch test the optical-fiber cable is folded over itself so that the two cable portions on either side of the fold are substantially parallel to one another, thereby creating a pinch in the optical-fiber cable. The resulting folded cable has a maximum diameter of about twice the cable's normal diameter. At the pinch point, the optical-fiber cable is flattened and has a diameter less than its normal diameter. Although the optical-fiber cable has a reduced diameter at and near the pinch point, the remainder of the cable retains its normal diameter. The cable is held in this pinched state for about 10 minutes. After 10 minutes have elapsed and while the cable is held in the pinched state, the attenuation of the optical fibers in the cable is measured (e.g., at room temperature).

During a pinch test at 850 nm, exemplary multimode optical fibers accordance with the present invention (and contained within an optical-fiber cable) typically have attenuation added losses of less than about 0.5 dB, more typically less than about 0.3 dB (e.g., less than about 0.25 dB, such as less than 0.20 dB).

Exemplary optical-fiber cables capable of excellent pinch-test performance are disclosed in commonly assigned U.S. patent application Ser. No. 13/401,026 for an Optical Fiber Interconnect Cable, which is hereby incorporated by reference in its entirety.

The present optical fibers may facilitate the reduction in overall optical-fiber diameter. As will be appreciated by those having ordinary skill in the art, a reduced-diameter optical fiber is cost-effective, requiring less raw material. Moreover, a reduced-diameter optical fiber requires less deployment space (e.g., within a buffer tube and/or fiber optic cable), thereby facilitating increased fiber count and/or reduced cable size.

Those having ordinary skill in the art will recognize that an optical fiber with a primary coating (and an optional secondary coating and/or ink layer) typically has an outer diameter of between about 235 microns and about 265 microns (μm). The component glass fiber itself (i.e., the glass core and surrounding cladding layers) typically has a diameter of about 125 microns, such that the total coating thickness is typically between about 55 microns and 70 microns.

With respect to the present optical fiber, the component glass fiber typically has an outer diameter of about 125 microns. With respect to the optical fiber's surrounding coating layers, the primary coating typically has an outer diameter of between about 175 microns and about 195 microns (i.e., a primary coating thickness of between about 25 microns and 35 microns), and the secondary coating typically has an outer diameter of between about 235 microns and about 265 microns (i.e., a secondary coating thickness of between about 20 microns and 45 microns). Optionally, the present optical fiber may include an outermost ink layer, which is typically between two and ten microns in thickness.

In one alternative embodiment, an optical fiber may possess a reduced diameter (e.g., an outermost diameter between about 150 microns and 230 microns). In this alternative optical fiber configuration, the thickness of the primary coating and/or secondary coating is reduced, while the diameter of the component glass fiber is maintained at about 125 microns. (Those having ordinary skill in the art will appreciate that, unless otherwise specified, diameter measurements refer to outer diameters.)

By way of illustration, in such exemplary embodiments, the primary coating layer may have an outer diameter of between about 135 microns and about 175 microns (e.g., about 160 microns), typically less than 165 microns (e.g., between about 135 microns and 150 microns), and usually more than 140 microns (e.g., between about 145 microns and 155 microns, such as about 150 microns).

Moreover, in such exemplary embodiments, the secondary coating layer may have an outer diameter of between about 150 microns and about 230 microns (e.g., more than about 165 microns, such as 190-210 microns or so), typically between about 180 microns and 200 microns. In other words, the total diameter of the optical fiber is reduced to less than about 230 microns (e.g., between about 195 microns and 205 microns, and especially about 200 microns). By way of further illustration, an optical fiber may employ a secondary coating of about 197 microns at a tolerance of +/−5 microns (i.e., a secondary-coating outer diameter of between 192 microns to 202 microns). Typically, the secondary coating will retain a thickness of at least about 10 microns (e.g., an optical fiber having a reduced thickness secondary coating of between 15 microns and 25 microns).

In another alternative embodiment, the outer diameter of the component glass fiber may be reduced to less than 125 microns (e.g., between about 60 microns and 120 microns), perhaps between about 70 microns and 115 microns (e.g., about 80-110 microns). This may be achieved, for instance, by reducing the thickness of one or more cladding layers. As compared with the prior alternative embodiment, (i) the total diameter of the optical fiber may be reduced (i.e., the thickness of the primary and secondary coatings are maintained in accordance with the prior alternative embodiment) or (ii) the respective thicknesses of the primary and/or secondary coatings may be increased relative to the prior alternative embodiment (e.g., such that the total diameter of the optical fiber might be maintained).

By way of illustration, with respect to the former, a component glass fiber having a diameter of between about 90 and 100 microns might be combined with a primary coating layer having an outer diameter of between about 110 microns and 150 microns (e.g., about 125 microns) and a secondary coating layer having an outer diameter of between about 130 microns and 190 microns (e.g., about 155 microns). With respect to the latter, a component glass fiber having a diameter of between about 90 and 100 microns might be combined with a primary coating layer having an outer diameter of between about 120 microns and 140 microns (e.g., about 130 microns) and a secondary coating layer having an outer diameter of between about 160 microns and 230 microns (e.g., about 195-200 microns).

Reducing the diameter of the component glass fiber might make the resulting optical fiber more susceptible to microbending attenuation. That said, the advantages of further reducing optical-fiber diameter might be worthwhile for some optical-fiber applications.

As noted, the present optical fibers may include one or more coating layers (e.g., a primary coating and a secondary coating). At least one of the coating layers—typically the secondary coating—may be colored and/or possess other markings to help identify individual fibers. Alternatively, a tertiary ink layer may surround the primary and secondary coatings.

The present optical fibers may be manufactured by drawing from final preforms.

A final preform may be manufactured by providing a primary preform with an outer overcladding layer (i.e., an overcladding process). The outer overcladding layer typically consists of doped or undoped, natural or synthetic, silica glass. Several methods are available for providing the outer overcladding layer.

In a first exemplary method, the outer overcladding layer may be provided by depositing and vitrifying natural or synthetic silica particles on the outer periphery of the primary preform under the influence of heat. Such a process is known, for example, from U.S. Pat. Nos. 5,522,007, 5,194,714, 6,269,663, and 6,202,447, each of which is hereby incorporated by reference in its entirety.

In another exemplary method, a primary preform may be overcladded using a silica sleeve tube, which may or may not be doped. This sleeve tube may then be collapsed onto the primary preform.

In yet another exemplary method, an overcladding layer may be applied via an Outside Vapor Deposition (OVD) method. Here, a soot layer is first deposited on the outer periphery of a primary preform, and then the soot layer is vitrified to form glass.

The primary preforms may be manufactured via outside vapor deposition techniques, such as Outside Vapor Deposition (OVD) and Vapor Axial Deposition (VAD). Alternatively, the primary preforms may be manufactured via inside deposition techniques in which glass layers are deposited on the inner surface of a substrate tube of doped or undoped silica glass, such as Modified Chemical Vapor Deposition (MCVD), Furnace Chemical Vapor Deposition (FCVD), and Plasma Chemical Vapor Deposition (PCVD).

By way of example, the primary preforms may be manufactured using a PCVD process, which can precisely control the central core's gradient refractive index profile.

A depressed trench, for instance, may be deposited on the inner surface of a substrate tube as part of the chemical vapor deposition process. More typically, a depressed trench may be manufactured either (i) by using a fluorine-doped substrate tube as the starting point of the internal deposition process for deposition of the gradient refractive index central core or (ii) by sleeving a fluorine-doped silica tube over the gradient refractive index central core, which itself may be produced using an outside deposition process (e.g., OVD or VAD). Accordingly, a component glass fiber manufactured from the resulting preform may have a depressed trench located at the periphery of its central core.

As noted, a primary preform may be manufactured via an inside deposition process using a fluorine-doped substrate tube. The resulting tube containing the deposited layers may be sleeved by one or more additional fluorine-doped silica tubes so as to increase the thickness of a depressed trench, or to create a depressed trench having a varying refractive index over its width. Although not required, one or more additional sleeve tubes (e.g., fluorine-doped substrate tubes) may be collapsed onto the primary preform before an overcladding step is carried out. The process of sleeving and collapsing is sometimes referred to as jacketing and may be repeated to build several glass layers on the outside of the primary preform.

The present optical fibers may be deployed in various structures, such as those exemplary structures disclosed hereinafter.

For example, one or more of the present optical fibers may be enclosed within a buffer tube. For instance, optical fiber may be deployed in either a single-fiber loose buffer tube or a multi-fiber loose buffer tube. With respect to the latter, multiple optical fibers may be bundled or stranded within a buffer tube or other structure. In this regard, within a multi-fiber loose buffer tube, fiber sub-bundles may be separated with binders (e.g., each fiber sub-bundle is enveloped in a binder). Moreover, fan-out tubing may be installed at the termination of such loose buffer tubes to directly terminate loose buffered optical fibers with field-installed connectors.

In other embodiments, the buffer tube may tightly surround the outermost optical fiber coating (i.e., tight buffered fiber) or otherwise surround the outermost optical-fiber coating or ink layer to provide an exemplary radial clearance of between about 50 and 100 microns (i.e., a semi-tight buffered fiber).

With respect to the former tight buffered fiber, the buffering may be formed by coating the optical fiber with a curable composition (e.g., a UV-curable material) or a thermoplastic material. The outer diameter of tight buffer tubes, regardless of whether the buffer tube is formed from a curable or non-curable material, is typically less than about 1,000 microns (e.g., either about 500 microns or about 900 microns).

With respect to the latter semi-tight buffered fiber, a lubricant may be included between the optical fiber and the buffer tube (e.g., to provide a gliding layer).

As will be known by those having ordinary skill in the art, an exemplary buffer tube enclosing optical fibers as disclosed herein may be formed of polyolefins (e.g., polyethylene or polypropylene), including fluorinated polyolefins, polyesters (e.g., polybutylene terephthalate), polyamides (e.g., nylon), as well as other polymeric materials and blends. In general, a buffer tube may be formed of one or more layers. The layers may be homogeneous or include mixtures or blends of various materials within each layer.

In this context, the buffer tube may be extruded (e.g., an extruded polymeric material) or pultruded (e.g., a pultruded, fiber-reinforced plastic). By way of example, the buffer tube may include a material to provide high temperature and chemical resistance (e.g., an aromatic material or polysulfone material).

Although buffer tubes typically have a circular cross section, buffer tubes alternatively may have an irregular or non-circular shape (e.g., an oval or a trapezoidal cross-section).

Alternatively, one or more of the present optical fibers may simply be surrounded by an outer protective sheath or encapsulated within a sealed metal tube. In either structure, no intermediate buffer tube is necessarily required.

Multiple optical fibers as disclosed herein may be sandwiched, encapsulated, and/or edge bonded to form an optical fiber ribbon. Optical fiber ribbons can be divisible into subunits (e.g., a twelve-fiber ribbon that is splittable into six-fiber subunits). Moreover, a plurality of such optical fiber ribbons may be aggregated to form a ribbon stack, which can have various sizes and shapes.

For example, it is possible to form a rectangular ribbon stack or a ribbon stack in which the uppermost and lowermost optical fiber ribbons have fewer optical fibers than those toward the center of the stack. This construction may be useful to increase the density of optical elements (e.g., optical fibers) within the buffer tube and/or cable.

In general, it is desirable to increase the filling of transmission elements in buffer tubes or cables, subject to other constraints (e.g., cable or mid-span attenuation). The optical elements themselves may be designed for increased packing density. For example, the optical fiber may possess modified properties, such as improved refractive-index profile, core or cladding dimensions, or primary-coating thickness and/or modulus, to improve microbending and macrobending characteristics.

By way of example, a rectangular ribbon stack may be formed with or without a central twist (i.e., a “primary twist”). Those having ordinary skill in the art will appreciate that a ribbon stack is typically manufactured with rotational twist to allow the tube or cable to bend without placing excessive mechanical stress on the optical fibers during winding, installation, and use. In a structural variation, a twisted (or untwisted) rectangular ribbon stack may be further formed into a coil-like configuration (e.g., a helix) or a wave-like configuration (e.g., a sinusoid). In other words, the ribbon stack may possess regular “secondary” deformations.

As will be known to those having ordinary skill in the art, such optical fiber ribbons may be positioned within a buffer tube or other surrounding structure, such as a buffer-tube-free cable. Subject to certain restraints (e.g., attenuation), it is desirable to increase the density of elements such as optical fibers or optical fiber ribbons within buffer tubes and/or optical fiber cables.

A plurality of buffer tubes containing optical fibers (e.g., loose or ribbonized fibers) may be positioned externally adjacent to and stranded around a central strength member. This stranding can be accomplished helically in one direction, known as “S” or “Z” stranding, or via Reverse Oscillated Lay stranding, known as “S-Z” stranding. Stranding about the central strength member reduces optical fiber strain when cable strain occurs during installation and use.

Those having ordinary skill in the art will understand the benefit of minimizing fiber strain for both tensile cable strain and longitudinal compressive cable strain during installation or operating conditions.

With respect to tensile cable strain, which may occur during installation, the cable will become longer while the optical fibers can migrate closer to the cable's neutral axis to reduce, if not eliminate, the strain being translated to the optical fibers. With respect to longitudinal compressive strain, which may occur at low operating temperatures due to shrinkage of the cable components, the optical fibers will migrate farther away from the cable's neutral axis to reduce, if not eliminate, the compressive strain being translated to the optical fibers.

In a variation, two or more substantially concentric layers of buffer tubes may be positioned around a central strength member. In a further variation, multiple stranding elements (e.g., multiple buffer tubes stranded around a strength member) may themselves be stranded around each other or around a primary central strength member.

Alternatively, a plurality of buffer tubes containing optical fibers (e.g., loose or ribbonized fibers) may be simply placed externally adjacent to the central strength member (i.e., the buffer tubes are not intentionally stranded or arranged around the central strength member in a particular manner and run substantially parallel to the central strength member).

Alternatively still, the present optical fibers may be positioned within a central buffer tube (i.e., the central buffer tube cable has a central buffer tube rather than a central strength member). Such a central buffer tube cable may position strength members elsewhere. For instance, metallic or non-metallic (e.g., GRP) strength members may be positioned within the cable sheath itself, and/or one or more layers of high-strength yarns (e.g., aramid or non-aramid yarns) may be positioned parallel to or wrapped (e.g., contrahelically) around the central buffer tube (i.e., within the cable's interior space). As will be understood by those having ordinary skill in the art, such strength yarns provide tensile strength to fiber optic cables. Likewise, strength members can be included within the buffer tube's casing.

Strength yarns may be coated with a lubricant (e.g., fluoropolymers), which may reduce unwanted attenuation in fiber optic cables (e.g., rectangular, flat ribbon cables or round, loose tube cables) that are subjected to relatively tight bends (i.e., a low bend radius). Moreover, the presence of a lubricant on strength yarns (e.g., aramid strength yarns) may facilitate removal of the cable jacketing by reducing unwanted bonding between the strength yarns and the surrounding cable jacket.

In other embodiments, the optical fibers may be placed within a slotted core cable. In a slotted core cable, optical fibers, individually or as a fiber ribbon, may be placed within pre-shaped helical grooves (i.e., channels) on the surface of a central strength member, thereby forming a slotted core unit. The slotted core unit may be enclosed by a buffer tube. One or more of such slotted core units may be placed within a slotted core cable. For example, a plurality of slotted core units may be helically stranded around a central strength member.

Alternatively, the optical fibers may also be stranded in a maxitube cable design, whereby the optical fibers are stranded around themselves within a large multi-fiber loose buffer tube rather than around a central strength member. In other words, the large multi-fiber loose buffer tube is centrally positioned within the maxitube cable. For example, such maxitube cables may be deployed in optical ground wires (OPGW).

In another cabling embodiment, multiple buffer tubes may be stranded around themselves without the presence of a central member. These stranded buffer tubes may be surrounded by a protective tube. The protective tube may serve as the outer casing of the fiber optic cable or may be further surrounded by an outer sheath. The protective tube may either tightly surround or loosely surround the stranded buffer tubes.

As will be known to those having ordinary skill in the art, additional elements may be included within a cable core. For example, copper cables or other active, transmission elements may be stranded or otherwise bundled within the cable sheath. Passive elements may also be placed within the cable core, such as between the interior walls of the buffer tubes and the enclosed optical fibers. Alternatively and by way of example, passive elements may be placed outside the buffer tubes between the respective exterior walls of the buffer tubes and the interior wall of the cable jacket, or within the interior space of a buffer-tube-free cable.

For example, yarns, nonwovens, fabrics (e.g., tapes), foams, or other materials containing water-swellable material and/or coated with water-swellable materials (e.g., including super absorbent polymers (SAPs), such as SAP powder) may be employed to provide water blocking and/or to couple the optical fibers to the surrounding buffer tube and/or cable jacketing (e.g., via adhesion, friction, and/or compression). Exemplary water-swellable elements are disclosed in commonly assigned U.S. Pat. No. 7,515,795 for a Water-Swellable Tape, Adhesive-Backed for Coupling When Used Inside a Buffer Tube, which is hereby incorporated by reference in its entirety.

Moreover, an adhesive (e.g., a hot-melt adhesive or curable adhesive, such as a silicone acrylate cross-linked by exposure to actinic radiation) may be provided on one or more passive elements (e.g., water-swellable material) to bond the elements to the buffer tube. An adhesive material may also be used to bond the water-swellable element to optical fibers within the buffer tube. Exemplary arrangements of such elements are disclosed in commonly assigned U.S. Pat. No. 7,599,589 for a Gel-Free Buffer Tube with Adhesively Coupled Optical Element, which is hereby incorporated by reference in its entirety.

The buffer tubes (or buffer-tube-free cables) may also contain a thixotropic composition (e.g., grease or grease-like gels) between the optical fibers and the interior walls of the buffer tubes. For example, filling the free space inside a buffer tube with water-blocking, petroleum-based filling grease helps to block the ingress of water. Further, the thixotropic filling grease mechanically (i.e., viscously) couples the optical fibers to the surrounding buffer tube.

Such thixotropic filling greases are relatively heavy and messy, thereby hindering connection and splicing operations. Thus, the present optical fibers may be deployed in dry cable structures (i.e., grease-free buffer tubes).

Exemplary buffer tube structures that are free from thixotropic filling greases are disclosed in commonly assigned U.S. Pat. No. 7,724,998 for a Coupling Composition for Optical Fiber Cables (Parris et al.), which is hereby incorporated by reference in its entirety. Such buffer tubes employ coupling compositions formed from a blend of high-molecular weight elastomeric polymers (e.g., about 35 weight percent or less) and oils (e.g., about 65 weight percent or more) that flow at low temperatures. Unlike thixotropic filling greases, the coupling composition (e.g., employed as a cohesive gel or foam) is typically dry and, therefore, less messy during splicing.

As will be understood by those having ordinary skill in the art, a cable enclosing optical fibers as disclosed herein may have a sheath formed from various materials in various designs. Cable sheathing may be formed from polymeric materials such as, for example, polyethylene, polypropylene, polyvinyl chloride (PVC), polyamides (e.g., nylon), polyester (e.g., PBT), fluorinated plastics (e.g., perfluorethylene propylene, polyvinyl fluoride, or polyvinylidene difluoride), and ethylene vinyl acetate. The sheath and/or buffer tube materials may also contain other additives, such as nucleating agents, flame-retardants, smoke-retardants, antioxidants, UV absorbers, and/or plasticizers.

The cable sheathing may be a single jacket formed from a dielectric material (e.g., non-conducting polymers), with or without supplemental structural components that may be used to improve the protection (e.g., from rodents) and strength provided by the cable sheath. For example, one or more layers of metallic (e.g., steel) tape, along with one or more dielectric jackets, may form the cable sheathing. Metallic or fiberglass reinforcing rods (e.g., GRP) may also be incorporated into the sheath. In addition, aramid, fiberglass, or polyester yarns may be employed under the various sheath materials (e.g., between the cable sheath and the cable core), and/or ripcords may be positioned, for example, within the cable sheath.

Similar to buffer tubes, optical fiber cable sheaths typically have a circular cross section, but cable sheaths alternatively may have an irregular or non-circular shape (e.g., an oval, trapezoidal, or flat cross-section).

By way of example, the present optical fiber may be incorporated into single-fiber drop cables, such as those employed for Multiple Dwelling Unit (MDU) applications. In such deployments, the cable jacketing must exhibit crush resistance, abrasion resistance, puncture resistance, thermal stability, and fire resistance as required by building codes. An exemplary material for such cable jackets is thermally stable, flame-retardant polyurethane (PUR), which mechanically protects the optical fibers yet is sufficiently flexible to facilitate easy MDU installations. Alternatively, a flame-retardant polyolefin or polyvinyl chloride sheath may be used.

In general, and as will be known to those having ordinary skill in the art, a strength member is typically in the form of a rod or braided/helically wound wires or fibers, though other configurations will be within the knowledge of those having ordinary skill in the art.

Optical fiber cables containing optical fibers as disclosed may be variously deployed, including as drop cables, distribution cables, feeder cables, trunk cables, and stub cables, each of which may have varying operational requirements (e.g., temperature range, crush resistance, UV resistance, and minimum bend radius).

Such optical fiber cables may be installed within ducts, microducts, plenums, or risers. By way of example, an optical fiber cable may be installed in an existing duct or microduct by pulling or blowing (e.g., using compressed air). An exemplary cable installation method is disclosed in commonly assigned U.S. Pat. No. 7,574,095 for a Communication Cable Assembly and Installation Method, (Lock et al.), and U.S. Pat. No. 7,665,902 for a Modified Pre-Ferrulized Communication Cable Assembly and Installation Method, (Griffioen et al.), each of which is incorporated by reference in its entirety.

As noted, buffer tubes containing optical fibers (e.g., loose or ribbonized fibers) may be stranded (e.g., around a central strength member). In such configurations, an optical fiber cable's protective outer sheath may have a textured outer surface that periodically varies lengthwise along the cable in a manner that replicates the stranded shape of the underlying buffer tubes. The textured profile of the protective outer sheath can improve the blowing performance of the optical fiber cable. The textured surface reduces the contact surface between the cable and the duct or microduct and increases the friction between the blowing medium (e.g., air) and the cable. The protective outer sheath may be made of a low coefficient-of-friction material, which can facilitate blown installation. Moreover, the protective outer sheath can be provided with a lubricant to further facilitate blown installation.

In general, to achieve satisfactory long-distance blowing performance (e.g., between about 3,000 to 5,000 feet or more), the outer cable diameter of an optical fiber cable should be no more than about 70 to 80 percent of the duct's or microduct's inner diameter.

Compressed air may also be used to install optical fibers in an air blown fiber system. In an air blown fiber system, a network of unfilled cables or microducts is installed prior to the installation of optical fibers. Optical fibers may subsequently be blown into the installed cables as necessary to support the network's varying requirements.

Moreover, the optical fiber cables may be directly buried in the ground or, as an aerial cable, suspended from a pole or pylon. An aerial cable may be self-supporting, or secured or lashed to a support (e.g., messenger wire or another cable). Exemplary aerial fiber optic cables include overhead ground wires (OPGW), all-dielectric self-supporting cables (ADSS), all dielectric lash cables (AD-Lash), and figure-eight cables, each of which is well understood by those having ordinary skill in the art. Figure-eight cables and other designs can be directly buried or installed into ducts, and may optionally include a toning element, such as a metallic wire, so that they can be found with a metal detector.

In addition, although the optical fibers may be further protected by an outer cable sheath, the optical fiber itself may be further reinforced so that the optical fiber may be included within a breakout cable, which allows for the individual routing of individual optical fibers.

To effectively employ the present optical fibers in a transmission system, connections are required at various points in the network. Optical fiber connections are typically made by fusion splicing, mechanical splicing, or mechanical connectors.

The mating ends of connectors can be installed to the optical fiber ends either in the field (e.g., at the network location) or in a factory prior to installation into the network. The ends of the connectors are mated in the field in order to connect the optical fibers together or connect the optical fibers to the passive or active components. For example, certain optical fiber cable assemblies (e.g., furcation assemblies) can separate and convey individual optical fibers from a multiple optical fiber cable to connectors in a protective manner.

The deployment of such optical fiber cables may include supplemental equipment, which itself may employ the present optical fiber as previously disclosed. For instance, an amplifier may be included to improve optical signals. Dispersion compensating modules may be installed to reduce the effects of chromatic dispersion and polarization mode dispersion. Splice boxes, pedestals, and distribution frames, which may be protected by an enclosure, may likewise be included. Additional elements include, for example, remote terminal switches, optical network units, optical splitters, and central office switches.

A cable containing the present optical fibers may be deployed for use in a communication system (e.g., networking or telecommunications). A communication system may include fiber optic cable architecture such as fiber-to-the-node (FTTN), fiber-to-the-telecommunications enclosure (FTTE), fiber-to-the-curb (FTTC), fiber-to-the-building (FTTB), and fiber-to-the-home (FTTH), as well as long-haul or metro architecture. Moreover, an optical module or a storage box that includes a housing may receive a wound portion of the optical fiber disclosed herein. By way of example, the optical fiber may be wound around a bending radius of less than about 15 millimeters (e.g., 10 millimeters or less, such as about 5 millimeters) in the optical module or the storage box.

Moreover, present optical fibers may be used in other applications, including, without limitation, fiber optic sensors or illumination applications (e.g., lighting).

The present optical fibers may include Fiber Bragg Grating (FBG). As will be known by those having ordinary skill in the art, FBG is a periodic or aperiodic variation in the refractive index of an optical fiber core and/or cladding. This variation in the refractive index results in a range of wavelengths (e.g., a narrow range) being reflected rather than transmitted, with maximum reflectivity occurring at the Bragg wavelength.

Fiber Bragg Grating is commonly written into an optical fiber by exposing the optical fiber to an intense source of ultraviolet light (e.g., a UV laser). In this respect, UV photons may have enough energy to break molecular bonds within an optical fiber, which alters the structure of the optical fiber, thereby increasing the optical fiber's refractive index. Moreover, dopants (e.g., boron or germanium) and/or hydrogen loading can be employed to increase photosensitivity.

In order to expose a coated glass fiber to UV light for the creation of FBG, the coating may be removed. Alternatively, coatings that are transparent at the particular UV wavelengths (e.g., the UV wavelengths emitted by a UV laser to write FBG) may be employed to render coating removal unnecessary. In addition, silicone, polyimide, acrylate, or PFCB coatings, for instance, may be employed for high-temperature applications.

A particular FBG pattern may be created by employing (i) a photomask placed between the UV light source and the optical fiber, (ii) interference between multiple UV light beams, which interfere with each other in accordance with the desired FBG pattern (e.g., a uniform, chirped, or titled pattern), or (iii) a narrow UV light beam for creating individual variations. The FBG structure may have, for example, a uniform positive-only index change, a Gaussian-apodized index change, a raised-cosine-apodized index change, or a discrete phase-shift index change. Multiple FBG patterns may be combined on a single optical fiber.

Optical fibers having FBG may be employed in various sensing applications (e.g., for detecting vibration, temperature, pressure, moisture, or movement). In this respect, changes in the optical fiber (e.g., a change in temperature) result in a shift in the Bragg wavelength, which is measured by a sensor. FBG may be used to identify a particular optical fiber (e.g., if the optical fiber is broken into pieces).

Fiber Bragg Grating may also be used in various active or passive communication components (e.g., wavelength-selective filters, multiplexers, demultiplexers, Mach-Zehnder interferometers, distributed Bragg reflector lasers, pump/laser stabilizers, and supervisory channels).

To supplement the present disclosure, this application incorporates entirely by reference the following commonly assigned patents, patent application publications, and patent applications: U.S. Pat. No. 4,838,643 for a Single Mode Bend Insensitive Fiber for Use in Fiber Optic Guidance Applications (Hodges et al.); U.S. Pat. No. 7,623,747 for a Single Mode Optical Fiber (de Montmorillon et al.); U.S. Pat. No. 7,587,111 for a Single-Mode Optical Fiber (de Montmorillon et al.); U.S. Pat. No. 7,356,234 for a Chromatic Dispersion Compensating Fiber (de Montmorillon et al.); U.S. Pat. No. 7,483,613 for a Chromatic Dispersion Compensating Fiber (Bigot-Astruc et al.); U.S. Pat. No. 7,526,177 for a Fluorine-Doped Optical Fiber (Matthijsse et al.); U.S. Pat. No. 7,555,186 for an Optical Fiber (Flammer et al.); U.S. Pat. No. 8,055,111 for a Dispersion-Shifted Optical Fiber (Sillard et al.); U.S. Pat. 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To supplement the present disclosure, this application further incorporates entirely by reference the following commonly assigned patents, patent application publications, and patent applications: U.S. Pat. No. 5,574,816 for Polypropylene-Polyethylene Copolymer Buffer Tubes for Optical Fiber Cables and Method for Making the Same; U.S. Pat. No. 5,717,805 for Stress Concentrations in an Optical Fiber Ribbon to Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No. 5,761,362 for Polypropylene-Polyethylene Copolymer Buffer Tubes for Optical Fiber Cables and Method for Making the Same; U.S. Pat. No. 5,911,023 for Polyolefin Materials Suitable for Optical Fiber Cable Components; U.S. Pat. No. 5,982,968 for Stress Concentrations in an Optical Fiber Ribbon to Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No. 6,035,087 for an Optical Unit for Fiber Optic Cables; U.S. Pat. No. 6,066,397 for Polypropylene Filler Rods for Optical Fiber Communications Cables; U.S. Pat. No. 6,175,677 for an Optical Fiber Multi-Ribbon and Method for Making the Same; U.S. Pat. No. 6,085,009 for Water Blocking Gels Compatible with Polyolefin Optical Fiber Cable Buffer Tubes and Cables Made Therewith; U.S. Pat. No. 6,215,931 for Flexible Thermoplastic Polyolefin Elastomers for Buffering Transmission Elements in a Telecommunications Cable; U.S. Pat. No. 6,134,363 for a Method for Accessing Optical Fibers in the Midspan Region of an Optical Fiber Cable; U.S. Pat. No. 6,381,390 for a Color-Coded Optical Fiber Ribbon and Die for Making the Same; U.S. Pat. No. 6,181,857 for a Method for Accessing Optical Fibers Contained in a Sheath; U.S. Pat. No. 6,314,224 for a Thick-Walled Cable Jacket with Non-Circular Cavity Cross Section; U.S. Pat. No. 6,334,016 for an Optical Fiber Ribbon Matrix Material Having Optimal Handling Characteristics; U.S. Pat. No. 6,321,012 for an Optical Fiber Having Water Swellable Material for Identifying Grouping of Fiber Groups; U.S. Pat. No. 6,321,014 for a Method for Manufacturing Optical Fiber Ribbon; U.S. Pat. No. 6,210,802 for Polypropylene Filler Rods for Optical Fiber Communications Cables; U.S. Pat. No. 6,493,491 for an Optical Drop Cable for Aerial Installation; U.S. Pat. No. 7,346,244 for a Coated Central Strength Member for Fiber Optic Cables with Reduced Shrinkage; U.S. Pat. No. 6,658,184 for a Protective Skin for Optical Fibers; U.S. Pat. No. 6,603,908 for a Buffer Tube that Results in Easy Access to and Low Attenuation of Fibers Disposed Within Buffer Tube; U.S. Pat. No. 7,045,010 for an Applicator for High-Speed Gel Buffering of Flextube Optical Fiber Bundles; U.S. Pat. No. 6,749,446 for an Optical Fiber Cable with Cushion Members Protecting Optical Fiber Ribbon Stack; U.S. Pat. No. 6,922,515 for a Method and Apparatus to Reduce Variation of Excess Fiber Length in Buffer Tubes of Fiber Optic Cables; U.S. Pat. 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In the specification and/or figures, typical embodiments of the invention have been disclosed. The present invention is not limited to such exemplary embodiments. The use of the term “and/or” includes any and all combinations of one or more of the associated listed items. The figures are schematic representations and so are not necessarily drawn to scale. Unless otherwise noted, specific terms have been used in a generic and descriptive sense and not for purposes of limitation. 

The invention claimed is:
 1. A multimode optical fiber, comprising: a central core surrounded by an outer optical cladding having a refractive index value n_(cl), said central core having (i) an outer radius r₁ of 36 microns or greater, (ii) a maximum refractive index value n₀, (iii) a graded-index profile with respect to said outer optical cladding, and (iv) a relative refractive index difference: ${\Delta = \frac{n_{0}^{2} - n_{cl}^{2}}{2\; n_{0}^{2}}};$ an inner cladding positioned between said central core and said outer cladding, said inner cladding having an outer radius r₂; and a depressed trench positioned between said inner cladding and said outer optical cladding, said depressed trench having an outer radius r₃, a refractive index difference Δn₃ with respect to said outer optical cladding, and a volume V₃; wherein $\frac{\sqrt{2\Delta}}{r_{1}}$ is 5.6×10⁻³ μm⁻¹ or greater.
 2. The optical fiber according to claim 1, wherein $\frac{\sqrt{2\Delta}}{r_{1}}$ is 5.7×10⁻³ μm⁻¹ or greater.
 3. The optical fiber according to claim 1, wherein $\frac{\sqrt{2\Delta}}{r_{1}}$ is 5.9×10⁻³ μm⁻¹ or greater.
 4. The optical fiber according to claim 1, wherein said depressed trench's volume V₃ is between −1710%-μm² and −635%-μm².
 5. The optical fiber according to claim 1, wherein said central core's outer radius r₁ is between about 39 microns and 41 microns.
 6. The optical fiber according to claim 1, wherein said central core's outer radius r₁ is 40 microns or greater.
 7. The optical fiber according to claim 1, wherein the difference r₂−r₁ between said inner cladding's outer radius r₂ and said central core's outer radius r₁ is between 1 micron and 3 microns.
 8. The optical fiber according to claim 1, wherein the difference r₃−r₂ between said depressed trench's outer radius r₃ and said inner cladding's outer radius r₂ is between 3 microns and 7 microns.
 9. The optical fiber according to claim 1, wherein the difference r₃−r₂ between said depressed trench's outer radius r₃ and said inner cladding's outer radius r₂ is less than 5 microns.
 10. The optical fiber according to claim 1, wherein: said central core's outer radius r₁ is between 39 microns and 41 microns; the difference r₂−r₁ between said inner cladding's outer radius r₂ and said central core's outer radius r₁ is between 1 micron and 3 microns; the difference r₃−r₂ between said depressed trench's outer radius r₃ and said inner cladding's outer radius r₂ is between 3 microns and 7 microns; and wherein said depressed trench's volume V₃ is between −790%-μm² and −565%-μm².
 11. The optical fiber according to claim 1, wherein, for two turns around a radius of curvature of 5 millimeters at a wavelength of 850 nanometers, the optical fiber has bending losses of 0.3 dB or less.
 12. The optical fiber according to claim 1, wherein, for two turns around a radius of curvature of 5 millimeters at a wavelength of 850 nanometers, the optical fiber has bending losses of 0.2 dB or less.
 13. A multimode optical fiber, comprising: a central core surrounded by an outer optical cladding having a refractive index value n_(cl), said central core having (i) an outer radius r₁ of 35 microns or greater, (ii) a maximum refractive index value n₀, (iii) a graded-index profile with respect to said outer optical cladding, and (iv) a relative refractive index difference: ${\Delta = \frac{n_{0}^{2} - n_{cl}^{2}}{2\; n_{0}^{2}}};$ an inner cladding positioned between said central core and said outer cladding, said inner cladding having an outer radius r₂; and a depressed trench positioned between said inner cladding and said outer optical cladding, said depressed trench having an outer radius r₃, a refractive index difference Δn₃ with respect to said outer optical cladding, and a volume V₃; wherein $\frac{\sqrt{2\Delta}}{r_{1}}$ is 5.1×10⁻³ μm⁻¹ or greater; and wherein the difference r₃−r₂ between said depressed trench's outer radius r₃ and said inner cladding's outer radius r₂ is less than 5 microns.
 14. The optical fiber according to claim 13, wherein $\frac{\sqrt{2\Delta}}{r_{1}}$ is 5.4×10⁻³ μm⁻¹ or greater.
 15. The optical fiber according to claim 13, wherein said depressed trench's volume V₃ is between −1710%-μm² and −635%-μm².
 16. The optical fiber according to claim 13, wherein said central core's outer radius r₁ is 39 microns or greater.
 17. The optical fiber according to claim 13, wherein, for two turns around a radius of curvature of 5 millimeters at a wavelength of 850 nanometers, the optical fiber has bending losses of 0.3 dB or less.
 18. A multimode optical fiber, comprising: a central core surrounded by an outer optical cladding having a refractive index value n_(cl), said central core having (i) an outer radius r₁ of 35 microns or greater, (ii) a maximum refractive index value n₀, (iii) a graded-index profile with respect to said outer optical cladding, and (iv) a relative refractive index difference: ${\Delta = \frac{n_{0}^{2} - n_{cl}^{2}}{2\; n_{0}^{2}}};$ an inner cladding positioned between said central core and said outer cladding, said inner cladding having an outer radius r₂; and a depressed trench positioned between said inner cladding and said outer optical cladding, said depressed trench having an outer radius r₃, a refractive index difference Δn₃ with respect to said outer optical cladding, and a volume V₃; wherein said depressed trench's volume V₃ is between −1710%-μm² and −830%-μm²; wherein $\frac{\sqrt{2\Delta}}{r_{1}}$ is 5.1×10⁻³ μm⁻¹ or greater; and wherein the difference r₃−r₂ between said depressed trench's outer radius r₃ and said inner cladding's outer radius r₂ is between 3 microns and 7 microns.
 19. The optical fiber according to claim 18, wherein $\frac{\sqrt{2\Delta}}{r_{1}}$ is 5.6×10⁻³ μm⁻¹ or greater.
 20. The optical fiber according to claim 18, wherein said central core's outer radius r₁ is 39 microns or greater.
 21. The optical fiber according to claim 18, wherein, for two turns around a radius of curvature of 5 millimeters at a wavelength of 850 nanometers, the optical fiber has bending losses of 0.3 dB or less. 